The phytotoxicity of atrazine and metolachlor was tested using rhizome cultures of Scirpus olneyi, a major salt marsh emergent macrophyte that has wide distribution around Gulf estuaries. A variety of types of exposure media and methods of toxicant addition were employed. Test systems included: (1) rhizomes placed in biochambers with atrazine-spiked “clean” estuarine sediment; (2) rhizomes placed in biochambers containing composited estuarine sediment with grain sizes ranging from sands to clays; (3) young shoots placed in biochambers prepared as in (2); and; (4) young shoots placed in biochambers in seawater diluted to varying salinities. Metolachlor was the test pesticide in systems 2–4. Plant responses measured included peroxidase activity (POD), peroxidation products, chlorophyll, and growth. All responses to atrazine-spiked sediments were clearly related to the dose, whereas responses to metolachlor showed high variability with increasing salinity and low variability with varying grain size. At 12‰, salinity effects completely masked the metolachlor effects (as measured by growth) at all test levels, indicating that growth as an ecological endpoint used to evaluate a chemical stressor is ineffective under certain salinity regimes.